Electrocoating method



Dec. 19, 1950 Original Filed March 7, 1947 G. c. cox 2,534,234

ELECTROCOATING METHOD 2 Sheets-Sheet 1 Fig.1.

TEMPERATURE or 1 ECTROL v75 0 20 40 so so |00C .h

Fig. 2.

AMPS. PER Sal-'7.

INVENTOR. 660736 CT Uox A'ITDRNEY Dec. 19, 1950 G. c. cox 2,534,234

ELECTROCOATING METHOD Original Filed March 7, 1947 2 Sheets-Sheet 2 VO/DS CALCIUM RICH CRYSTALS g. .0

SURFACE BE/IVG COH ED Fig.4.

SI-IR INKA 6E CRACKS b @UUC;

SURFACE BEING COATED MAGNESIUM RICH AMORPHOUS COMPOUNDS Fig.5.

MAGNESIUM RICH AMORPHOUS MATRIX SURFACE BEING COATED uucnl'or George C. Cox

my wax/(4W Patented Dec. 19, 1950 UNITED STATES PATENT OFFICE Gontinuation; oi:- appiication Serial No. 733,213, March .7, 19. This application February 20,

: 1948, Serial No. 9,933

- 7 Claims.

Tins-invention may be manufactured andused by or for the Government for governmental pur poses, without the paymentito me-of royalty thereon.

This, invention relates to coating-methods, and more particularlyto the methods of electrocoating from anelectrolyte whichhas-been formed from the alkaline-earth metal salts, particularly their chlorides, sulphates or nitrates.

- (Granted under the act 'ofi' March 3, 1883, as amended April 30, 1928; 1370 0. G. 757) -This invention is an i-mproyementover the 10 invention. disclosed and claimed in. my Patent Number. 2,200,469 issued, May. 14, 1940.

..It .is.an-obje'ctl-of mylinven'tion-to. provide a novel method for producing a, corrosion resistant binary coating. of substantially uniform thick will" becomereadily apparent when thexfollowiiig a specification"zis' read in conjunction' with the. accompanying drawings wherein:

Figure: 1 is a. schematic view of an apparatus thatrmay be. utilizedufor carrying out myin-vention.

various. operating conditions of --my= electrocoating rmethod.

:Flgure 3 represents a highly'magnified cross- Figure 2 representsza series of graphs showing sectional view of" a coating formed under 0011-. 40

di-tions-to produce-a high calcium content.

J-Figure 4 representsa highlymagniiied crosssec-tionaL-yiew ofaa coating formed under conditions-to produce ahigh magnesium content.

.. Figure 5. representsra-highly magnified crossa sectional view of a coating formed in. accordance wlthsmy invention.

.My method may becarried out. by the useof eithena large orsmall body or. electrolyte. "In

general the electrolyte used is=-onecontaining magnesium.-and .-calcium salts. of; the chlorides, sulphateseorflnitrates. with. or withoufsimilar salts. of. theaalkali metals. "The magnesium and ca1cium;..concentration of f the. electrolyte. may

varywfrom. slighftsalinity, such; round in,"

gel type of matrix.

appears to depend to a large extent on'thequanbrackish waters, upward to i,nc1udethe cioncen trations found in sea water and even-more cone centrated solutions sothat a binary" non-niet'allic coating as hereinafter described-will befomid by-electrolysis within the temperature and current density ranges herein set forth.

l Excellent, firmly adherent, non-metallioxprotecting coatings have been formedfrom;electrolyte of sea water orof concentrated sea-water when operating-at current densities above-9'10 ampere per square foot and'und-er conditions dee lineated by the curves-Ato E of Fig- 2. 'I-herefore by use of Sea water or the brines-madearom inland salt. deposits (which have-beenformed from sea Water) an-inexpensivesource of raw materials is availableat almost any: location throughout the worldreplacement costs inherent in such electroplating processes is also eliminated. Various lakesha vev concentrated brines which can be-used-as-a source of raw materials.

It appears that. the salts of the alkaline earth metals. are essential to the-formation ofthese twocomponent coatings.

calcium salts of the chlorides, sulphatesornitrates. Various mixtures of these salts may also be used provided that. the electrolyte contains.

magnesium and calcium. ions'in sufiicientquantityto cause the, desired plating action: to take It appears that the essential formation of a colloidal bonding agent is dependentv upon.

place.

the presence of magnesium ions in'solutionin sufiicient quantity for the initial deposition of a The hardness of a coating tity of the calcium ions which are discharged from solution. The ductility of. an J electrodeposited coating is considerably improvedby the presence of ions of the alkalies such as obtained by the addition ofsodium' orpota ssium chlorides, sulphates, nitrates, etc. "The hydrogen ion concentration can be variedover quite wide ranges, for example, useful coatings have been deposited from synthetic sea Water and sim.

ilar electrolytes of the chlorides, sulphates or nitrates of the alkaline earth metals with pH ranges as low as 4.0 and as high as 9.5. However, come of the coatings with-thegreatest density and best. protective qualities have been formed at values between L5 and 5.5- on theIp'H scale, particularly, when sodium chloride was added to the solution. It appears that the 61 .16: tility of an' electroadeposited coating is improved The usual -mixing and.

Extremely dense coatings having excellent adherent properties have been formed from aqueous solutions of magnesium and Gms. per liter Sodium chloride, about 26.5

Magnesium chloride, about 3.2 Magnesium sulphate, about 2.1 Calcium sulphate, about"- 1.4 Other salts, possibly 1.5

In terms of ion content these values are approximately Gms. per liter Sodium ions, about 10.5 Magnesium ions, about 1.2 Calcium ions, about 0.41 Chloride ions, about 18.38 Sulphate ions, about 2.6

Useful coatings have been formed when substituting chloride ion equivalent to the total equivalent non-metal negative ion content.

Similarly, this total non-metal ion equivalent can be made up from other salts of the strong acids such as the sulphates or nitrates.

Whereas the process set forth in my prior Patent Number 2,200,469 required a period of time from about 24 to '72 hours to form a suitable deposit, coatings made by the process to be herein disclosed and claimed may be formed within a period of seconds to permit the efficient, economical and high speed coating of continuous wire, strip or plate. For either continuous or intermittent modern high speed coating operations it is generally desirable to use electrolytes containing the maximum quantity of magnesium ions to which is added the maximum quantity of calcium ions that will remain in solution. The use of my method permits binary coatings of a type to be more fully described to be quickly and easily applied in a manner that was impossible to attain previous to this development.

Referring to Figure 1, a tank I! holds therein an electrolyte I ll containing magnesium, other alkaline earth ions and ions of an alkali. The object to be coated i4, is connected to a low voltage direct current source of electricity. The electrode 15 may be covered with a substance it such, for example, as dolomite that will react with detrimental anode products which may be liberated during the electrolysis. The tank I! is generally heated by means of a heating element I8 such as a steam coil or other equivalent means. For automatic operation a thermostat II is fixed to the side of tank I! and controls the heating unit I B in accordance with the desired electrolyte temperature. 2

In carrying out my coating method, I have found that a definite critical relationship exists between the cathode current density and the temperature of the electrolyte. This relationship may be expressed by the equation T=34 logroX-l-C' wherein T is the temperature of the electrolyte in degrees centigrade, X is the cathode current density in amperes per square foot, and C is a constant within the range of B to 70.

As far as can be determined, the coating resulting from the utilization of an electrolyte containing magnesium and calcium ions com- 4 prises substantially uniformly dispersed microscopic crystals and crystal aggregates which compounds of calcium in the substantially pure form. These crystals are embedded in an amorphous matrix which fills the microscopic voids around the crystals so that the porosity is reduced to an extremely low value. As far as can be determined, the magnesium content of this coating composition is substantially contained in the matrix in the form of various low soluble magnesium compounds as the hydroxide, oxides, oxy-chloride, or basic carbonate, etc. It appears that this matrix is an amorphous or colloidal mixture containing the magnesium compounds with or Without some of the calcium constituents.

I have found that depending on the range of current density and electrolyte temperature, the deposited coating can be made to vary from a high magnesium content, gelatinous or amorphous type of coating, to a high calcium content, highly granular, crystalline type of coating. The gelatinous type of coating is unsuitable for some applications because it may be easily washed off. On the other hand the high calcium content granular type of coating may be unsuitable for some applications since the coating contains voids between the crystals that will permit corrosive substances to penetrate the coating.

Referring to Figure 2, the five curves A, B, C, D, and E represent different conditions of electrode current density and electrolyte temperature. Operating conditions outside of the area bounded by curves A and E will result in unsatisfactory coatings. Operation below the conditions of curve A will result in a gelatinous coating having a high magnesium content, the properties of which have already been described. Operating conditions above those conditions on curve E will result in a high-calcium, highly granular, crystalline type of coating, the proper ties of which have already been described. The area shown between curves A and E represents the operating conditions that will result in the best coatings.

Operation of my method above the curve E of Figure 2 will result in the type of coating illustrated in Figure 3. This coating consists essentially of microscopic calcium crystals depositedcoating when dry contains shrinkage cracks which permit portions of the surface to be exposed. Such a deposit is not useful as a corrosion resistant coating or as a dielectric.

If the conditions of temperature and current density between curves A and E of Figure 2 are utilized, a coating such as disclosed in Figure 5 will result with the best coatings resulting from operating conditions of electrolyte temperature and current density falling between the curves B and C. In this coating, there is an abundant quantity of microscopic crystals of a calcium constituent, imbedded in an amorphous matrix con sist'lng essentially of magnesium compounds. Such a coating is tough, resilient and is excellent for the purpose of providing corrosion protection. Such a coating also forms an excellent dielectric.

When using electrolytes of sea water or syn" thetic solutions of these salts as defined above in which the magnesium and calcium ion content is at least equal to that found in sea water and those electrolytes containing a concentration of magnesium and calcium ions sufficient to cause plating action when operating under the conditions delineated within the area of the graphs A to E inclusive, it is interesting to note the calcium and magnesium values in the electroeoatings which have been formed. These values are approximately as follows: Graph A has a magnesium equivalent of about 31% and a calcium equivalent of about 4%. On the other hand, graph E has a magnesium equivalent of about 2% and a calcium equivalent of about 34%. Graph C has a magnesium and calcium equivalent of approximately and 18%, respectively. Graph B has values somewhere between those of graph A and C and graph D has values somewhere between graph C and D. Because of difficulty in separately analyzing these two elements of the alkaline-earth group, applicant does not wish to be limited to a definition of these curves in terms of either chemical content but he prefers to define the electrocoatings formed in terms of the physical characteristics as herein discussed. It should be noted that when operating as escribed a non-metallic coating can be formed. which has characteristics approximating those of Fig. 5 or any variation thereof such as the poor deposits illustrated in Fig, 3 and Fig. 4.

In those instances wherein it is desired to coat a continuous wire, rod or strip wherein such wire, rod or strip is to be coated at a rate up to 600 feet per minute, my method ma be carried out by utilizing an electrolyte temperature of 80 (le grees C. and a current density of 10 amperes or more per square foot. A coating bath of suflicient extent to permit immersion of the article being coated for about six seconds would be sufficient to form a highly useful non-metallic coating of the type described. If, for example, it were desired to form such a non-metallic coating in three seconds, an electrolyte temperature of 90 degrees C. and a current density of 20 amperes per square foot could be utilized. At the higher current densities excellent flash-type coatings have been formed in as short a time as 1 seconds.

This application is a continuation of my copending application Serial No. 733,213 filed Mar. 7, 1947 (now abandoned), which in turn is a continuation-in-part of my co-pending application Serial No. 526,621 filed Mar. 15, 1944 (now abancloned).

I claim:

1. The process of producing a two component protective coating on a metal surface which comprises the steps of submerging that part of the metal surface to be coated in an aqueous electrolyte, maintaining said part of the metal surface cathodic, maintaining the effective current density on the cathodic metal surface greater than 0.4 ampere per square foot, maintaining, the cation content of said electrolyte to consist essentially of magnesium ions, other alkaline earth ions, and ions of an alkali in sufficient quantity to cause plating action to occur, maintaining the pH factor of said electrolyte within the range between the limits of 4.0 and 9.5, and maintain ing the temperature of the electrolyte below the boiling point under the operating conditions and within the range expressed by the equations T=34 10g10 X+30 and T=34 logio X+70, where T is the temperature in degrees centigrade and X is the effective current density on the cathodic metal surface in amperes per square foot.

2. The process of producing a protective coating on a metal surface which comprises the steps of immersing that part of the metal surface to be coated in an electrolyte, maintaining said part of the metal surface cathodic, maintaining the cathodic current density greater than 0.4 ampere per square foot, maintaining the cation content of said electrolyte to consist essentially of the ions of magnesium, calcium and sodium in concentrations at least equal to those found in sea water of each of said elements and a sulfate ion concentration substantially equivalent to the cation content, maintaining the pI-I factor of said electrolyte between the limits of 4.0 and 9.5, and maintaining the temperature of the electrolyte below the boiling point under the operating conditions and within the range defined by the equations T 34 logic X+30 and T=34 logm X+70, where T is the temperature in degrees centigrade and X is the cathodic current density in amperes per square foot.

3. The process of producing a protective coating on a metal surface which comprises the steps of immersing that part of the metal surface to be coated in an electrolyte, maintaining said part of the metal surface cathodic, maintaining the current density at the cathode greater than 0.4. ampere per square foot, maintaining the cation content of said electrolyte so as to consist essentially of the ions of magnesium, calcium and sodium in concentrations at least equal to those found in sea water of each of said elements and a chloride ion concentration substantially equivalent to the cation content, maintaining the pH factor of said electrolyte within the range between the limits of 4.0 and 9.5, and maintaining the temperature of the electrolyte below the boiling point under the operating conditions of pressure and ion concentration and within the range defined by the equations T=34 logm X+30 and T=34 logio X+'70, where T is the temperature in degrees centigrade and X is the current density at the cathode in amperes per square foot.

4. The process of producing a protective coating on a metal surface which comprises the steps of immersing that part of the metal surface to be coated in an electrolyte, maintaining said part of the metal surface cathodic, maintaining the cathodic current density greater than 0.4 ampere per square foot, maintaining the cation content of said electrolyte to consist essentially of the ions of magnesium, calcium and sodium in concentrations at least equal to those found in sea water of each of said elements and a nitrate ion concentration substantially equivalent to the cation content, maintaining the pH factor of said electrolyte between the limits of 4.0 and 9.5, and maintaining the temperature of the electrolyte below the boiling point under the operation conditions and within the range defined by the equations T=34 logio X+30 and T=34 logic X+'70, where T is the temperature in degrees centigrade and X is the cathodic current density in amperes per square foot,

5. The process of producing a protective coating on a metal surface which comprises the steps of immersing that part of the surface to be coated in an electrolyte, maintaining said part of the metal surface cathodic, maintaining the cathodic current density greater than 0.4 ampere per square foot, maintaining the cation content of said electrolyte to consist essentially of the ions of magnesium, calcium and sodium in concentrations at least equal to those found in sea water of each of said elements and an anion content of chlorides, sulfates and nitrates, said anion concentration being substantially equivalent to the cation content, maintaining the pH factor of said electrolyte between the limits of 4.0 and 9.5, and maintaining the temperature of the electrolyte below the boiling point under the operating conditions and within the range defined by the equations T=34 10g10 X +30 and where T is the temperature in degrees centigrade and X is the cathodic current density in amperes per square foot.

6. The process of producing a protective coating on a metal surfac which comprises the steps of immersing that part of the metal surface to be coated in an electrolyte, maintaining said part of the metal surface cathodic, maintaining the cathodic current density greater than 0.4 ampere per square foot, maintaining said electrolyte to comprise essentially an aqueous solution providing ions of magnesium and calcium to be electrolytically deposited, maintaining the pH factor of said electrolyte within the range between the limits of 4.0 and 9.5, and maintaining the temperature of the electrolyte below the boiling point under the operating conditions and within the range expressed by the equations T=34 logic X+3O '8 and T=34 10g10 X+70, where T is the temperature in degrees centigrade and X is the cathodic current density in amperes per square foot.

7. The process of producing a protective coating on a metal surface which comprises the steps of immersing that part of the surface to be coated in an electrolyte, maintaining said part of the metal surface cathodic, maintaining the cathodic current density greater than 0.4 ampere per square foot, maintaining said electrolyt to consist of an aqueous solution having as the essential plating components magnesium and calcium ions in sufficient quantities for plating action to occur, maintaining the pH factor of said electrolyte within the range between the limits of 4.0 and 9.5, and maintaining the temperature of the electrolyte below the boiling point under the operating conditions and within the range expressed by the equations T:34 logic X+30 and T:34. 10g10 X+70, Where T is the temperature in degrees centigrade and X is the cathodic current density in amperes per square foot.

GEORGE C. COX.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 872,759 Schoneberger et a1. Dec. 3, 1907 2,200,469 Cox May 14, 1940 FOREIGN PATENTS Number Country Date 6,495 Great Britain of 1894 

1. THE PROCESS OF PRODUCING A TWO COMPONENT PROTECTIVE COATING ON A METAL SURFACE WHICH COMPRISES THE STEPS OF SUBMERGING THAT PART OF THE METAL SURFACE TO BE COATED IN AN AQUEOUS ELECTROLYTE, MAINTAINING SAID PART OF THE METAL SURFACE CATHODIC, MAINTAINING THE EFFECTIVE CURRENT DENSITY ON THE CATHODIC METAL SURFACE GREATER THAN 0.4 AMPERE PER SQUARE FOOT, MAINTAINING THE CATION CONTENT OF SAID ELECTROLYTE TO CONSIST ESSENTIALLY OF MAGNESIUM IONS, OTHER ALKALINE EARTH IONS, AND IONS OF AN ALKALI IN SUFFICIENT QUANTITY TO CAUSE PLATING ACTION TO OCCUR, MAINTAINING THE PH FACTOR OF SAID ELECTROLYTE WITHIN THE RANGE BETWEEN THE LIMITS OF 4.0 AND 9.5 AND MAINTAINING THE TEMPERATURE OF THE ELECTROLYTE BELOW THE BOILING POINT UNDER THE OPERATING CONDITIONS AND WITHIN THE RANGE EXPRESSED BY THE EQUATIONS T=34 LOG10 X+30 AND T=34 LOG12 X+70, WHERE T IS THE TEMPERATURE IN DEGREES CENTIGRADE AND X IS THE EFFECTIVE CURRENT DENSITY ON THE CATHODIC METAL SURFACE IN AMPERES PER SQUARE FOOT. 